The present invention is concerned with a process for the production of cellulose moulded bodies, and a regenerated, aqueous solution of an amine-oxide used for the production of a mouldable cellulose solution.
For some decades there has been searched for processes for the production of cellulose moulded bodies able to substitute the viscose process, today widely employed. As an alternative which is interesting among other reasons for its reduced environmental impact, a method has been found for dissolving cellulose without derivatisation in an organic solvent and extruding from this solution moulded bodies, e.g. fibres and films. Fibres thus extruded have received by BISFA (The International Bureau for the Standardization of man made fibers) the generic name Lyocell. By an organic solvent, BISFA understands a mixture of an organic chemical and water.
It has been shown that as an organic solvent, a mixture of a tertiary amine-oxide and water is particularly useful for the production of cellulose moulded bodies. As amine-oxide, basically N-methylmorpholine-N-oxide (NMMO) is used. Other amine-oxides are described e.g. in EP-A - 0 553 070. A method for the production of mouldable cellulose solutions is known e.g. from EP-A-0 365 419.
The cellulose is precipitated from the cellulose solution in an aqueous precipitation bath. During this process, amine-oxide builds up in the precipitation bath. To render the method economical, it is of decisive importance to recover and reuse nearly all of the amine-oxide. Thus the amine-oxide process presents the following 3 main steps:
(A) dissolving cellulose in an aqueous solution of a tertiary amine-oxide, in particular N-methylmorpholine-N-oxide (NMMO), to produce a mouldable cellulose solution, PA0 (B) moulding the cellulose solution and conducting the moulded cellulose solution into an aqueous precipitation bath, wherein the cellulose is precipitated, thus being obtained a moulded body and a spent precipitation bath, PA0 (C) regenerating, i.e. purifying and concentrating the spent precipitation bath, thus being obtained a regenerated aqueous amine-oxide solution which is used again in step (A) for dissolving cellulose. PA0 (A) dissolving cellulose in an aqueous solution of a tertiary amine-oxide, in particular N-methylmorpholine-N-oxide (NMMO), to produce a mouldable cellulose solution, PA0 (B) moulding the cellulose solution and conducting the moulded cellulose solution into an aqueous precipitation bath, wherein the cellulose is precipitated, thus being obtained a moulded body and a spent precipitation bath, PA0 (C) regenerating the spent precipitation bath, thus being obtained a regenerated aqueous amine-oxide solution which is used again in step (A) for dissolving cellulose,
The term "regenerating" is to be understood as any means capable of processing the precipitation bath so that an aqueous amine-oxide solution is produced, which can be used again in step (A). Such means include purification, treatment with ion exchangers, concentration, etc.
In the precipitation bath, not only amine-oxide but also degradation products of the cellulose and the amine-oxide build up. These may be heavily coloured, thus impairing the quality of the produced moulded bodies if they are not removed from the precipitation bath. Additionally, also metal traces may build up in the precipitation bath, leading to a reduced process safety.
In order to remove these degradation products, some proposals are known from the literature:
DD-A 254 199 describes a process for the purification of aqueous solutions of NMMO, according to which the solution passes through anion exchangers, wherein in a first step the anion exchanger contains an exchange resin of a styrene-divinylbenzene copolymerisate carrying tertiary amine groups of the -CH.sub.2 N(CH.sub.3).sub.2 type and in a second step quaternary ammonium groups of the -CH.sub.2 N(CH.sub.3).sub.3 OH type as functional groups. It is described that the NMMO solution to be purified is dark at the beginning of the purification, brown to yellow after the first step and bright yellow to transparent after the second step.
A disadvantage of this process consists in that the solutions thus treated exhibit a high pH value, which subsequently requires a more complex purification. Additionally, in this already known process alkali and earth alkali cations, as well as partially basic degradation products (morpholine, N-methylmorpholine and other compounds) are not removed from the solution. The metal ions and alkali and earth alkali metal ions respectively lead to undesired precipitations and incrustations, unwanted non-dissolved substances in the solution, and a reduced process safety. Although it is possible to remove these substances by adding a precipitation agent with subsequent filtration or other separating means, these operations however introduce additional chemicals or require additional technical processing.
EP-A - 0 427 701 describes a process for the purification of aqueous amine-oxide solutions, according to which the purification is carried out in a one-step process with an anion exchanger which as functional groups exclusively carries quarternary tetraalkyl ammonium groups of the formulas -CH.sub.2 N.sup.+ (CH.sub.3)X.sup.- or -CH.sub.2 N.sup.+ (CH.sub.3).sub.2 (CH.sub.2 OH)X.sup.-, X.sup.- representing the anion of an inorganic or organic acid, whereafter the anion exchanger is regenerated with an aqueous acidic solution. The anion X.sup.- preferably stems from a volatile acid, in particular carbonic acid, formic acid or acetic acid. These acids are also proposed for the regeneration of the anion exchanger.
In the International Patent Application WO93/11287 it is proposed to carry out the regeneration of the anion exchanger with an aqueous solution of a strong inorganic acid and subsequently with aqueous sodium hydroxide. It is further proposed to conduct the solution before or preferably after passing the anion exchanger through a cation exchanger. It is described that when employing a strong basic anion exchanger, the colouring of the exchanger resin produced by conducting the solution to be purified is so heavy that a mere regeneration with aqueous sodium hydroxide does not suffice to discolour the resin again. Therefore to maintain the capacity of the resin, it must be treated additionally with a strong inorganic acid.
The procedure described in WO93/11287 involves increased use of chemicals and obliges to use strong irritant substances, e.g. hydrochloric acid. Additionally it can be deduced from Example 5 of WO93/11287 that even when employing this process, the discolouring capacity of the anion exchanger after 10 operation cycles is reduced to nearly half of the original value.
A disadvantage of the amine-oxide process in contrast to the viscose process is the low thermal stability of the amine-oxides and in particular of the cellulose solution. This means that at the elevated process temperatures (approximately 110.degree.-120.degree. C.) there may be triggered off uncontrolled, highly exothermic decomposition processes in the cellulose solutions, which processes due to the development of gases may lead to heavy deflagrations or explosions and subsequently even to fires.
There is very little evidence found in the literature about the thermally unstable nature of the cellulose solution. The first clear reference to this phenomenon was made in 1986 by Buijtenhuijs et al. Especially in the presence of metal ions, the decomposition reactions in the spinning material may run away. On account of the metal construction of the plant components however, metal ions in the solution can never be excluded.
This runaway reaction cannot be prevented even by the addition of the stabiliser gallic acid propyl ester (GPE) widely used today (Buijtenhiujs et al., 1986). On the contrary, as studies have shown, GPE and also other aromatic hydroxy compounds having good complexing properties increase still further the thermal instability of the NMMO-cellulose solution in the presence of metals; this means that GPE (co)triggers the dangerous running away or the explosions. This is described in the Austrian Patent Application A 1857/93, published on Oct. 15th, 1994.
From U.S. Pat. No. 4,324,593, a process for the production of a mouldable solution containing cellulose dissolved in a solvent is known. The solvent contains a tertiary amine-oxide and a compound which increases the dissolution rate of the cellulose. As such compounds, particularly primary, secondary, tertiary amines, aqueous ammonium and alkali hydroxides are cited, tertiary amines being preferred. The authors of the patent suppose that the accelerating effect of these compounds is due primarily to the fact that they increase the pH of the solution. Evidence for the validity of this supposition however is not provided, and neither it is indicated which pH the solution should have. Only in Claim 27 of U.S. Pat. No. 4,324,593, a general reference is made to the fact that the accelerating compound should have a pH of more than 7, and in the Examples XIV and XV the pH of a mixture consisting of solid tertiary amine-oxide, cellulose and water with sodium hydroxide and aqueous ammonium respectively is adjusted to pH of 14 and 12,3 respectively.
In U.S. Pat. No. 4,324,593 it is proposed to add the accelerating compound to the solvent in such an amount that it represents up to 20% of the mass of the finished solution, suggesting that in each individual case the amount should depend on the amine-oxide used.
In order to prevent degradation of NMMO and cellulose, it is known from DD-A - 0 218 104 to add to the amine-oxide one or more basic substances in amounts of from 0.1 and 10 mole %, based on the cellulose solution. As basic substances, alkali hydroxides, e.g. NaOH, basically reacting salts, e.g. Na.sub.2 CO.sub.3, as well as organic nitrogen bases are recommended.